NX4D:
>> Coaxial cable has some very interesting properties which will allow two
>> completely different RF current flows on the shield, one on the inside, the
>> other on theoutside. This is how cancellation of radiation can take place
>> between the equal and opposite RF currents of the center conductor and the
>> inner surface of the shield, while a common mode RF current flows on the
>> outer surface of the shield.
>>
>> How a ferrite choke placed on the outside of the coax shield chokes only the
>> outer surface RF current flow, and not the inner, is beyond me, but it must
>> happen if the outer surface RF is choked while the inner RF continues to be
>> equal and opposite phase of the center conductor.
To get around this problem, get two pieces of low loss coax, place them side
by side and bond the shields together at each end. At each end each of the
parallel coax line, the parallel center conductors of the coax lines each
connect to one of the conductors of the open wire line transmission line.
You should then be able to wind the parallel coaxes around a toroid forming
common mode choke that won't attenuate the transmission line mode wave. Of
course, their will be mismatch associated with difference in characteristic
impedance between the open wire line and the parallel coax line. This needs
to be taken into account.
Mike, W4EF............
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